1. Field of the Invention
This invention relates generally to telecommunication systems, and, more particularly, to wireless telecommunication systems.
2. Description of the Related Art
The list of devices that use wireless communication techniques, such as a cellular telephone system and/or a Bluetooth-enabled wireless local area network (WLAN), to transmit voice and data signals has expanded dramatically in recent years to include, among other things, cell phones, personal data assistants, global positioning system receivers, laptop computers, and desktop computers. And the number of devices on the list, as well as the services they are likely to provide, is expected to continue to grow. The proliferation of wireless communication systems has led users to expect uninterrupted access to these systems and/or networks at virtually any time and in virtually any place.
Many wireless devices are able to communicate with a variety of different systems and/or networks. For example, a mobile phone may be able to establish a first wireless communication link with a base station in a cellular telephone system that operates according to the Universal Mobile Telephony System (UMTS) protocol while a user is outdoors. The same mobile phone may also be able to establish a second wireless communication link with an access point of a wireless local area network that operates according to the Bluetooth protocol when the user is in a “hot spot” associated with the access point. For another example, a mobile phone may be able to establish a first wireless communication link with a base station in a first cellular telephone system that operates according to the UMTS protocol while a user is in a first cell associated with the first base station. When in a second cell, the same mobile phone may also be able to establish a second wireless communication link with a second base station that operates according to a Global System for Mobile telecommunications (GSM) protocol.
Currently, voice communications are treated as fixed to a given system. For example, if a voice call is made on a GSM system, the voice call usually resides within the GSM domain for the duration of the call. It is unusual to handover the call to another system. However, if a handover from a first system to a second system, or between portions of the first system, is necessary or desirable, the handover is usually performed in a hard or “vertical” manner. For example, during a vertical handover from a GSM system to a UMTS system, the call is stopped on the GSM system and re-established on the UMTS system. For another example, during a hard handover from a first frequency to a second frequency of a UMTS system, the call stopped on the first frequency and re-established on the second frequency of the UMTS system. It should be noted that in both cases there is a complete break in the delivery of the voice as it is transferred to the new system or placed to another part of the same system.
Although vertical and/or hard handovers are typically considered adequate for conventional systems in which the handover is unlikely, e.g. handovers from GSM to UMTS systems, these conventional handover techniques have a number of disadvantages when applied to systems where voice calls may be handed over frequently, or even nearly continuously. For example, considerations such as convenience and price of delivery may make it desirable to handover voice calls frequently, or nearly continuously, in so-called “network of network” philosophies that will appear in 4th Generation wireless systems, which include many systems that compete (or collaborate) to deliver voice service. However, if a real time service (such as voice) is running at the time of the handover, the service will be impeded because of the discontinuity that arises when the service is stopped and then re-established. The discontinuity may become a time disconnect that could result in the call being interrupted or dropped.
The first and second systems participating in the handover may be classified according to the relative “tightness” or “looseness” of the coupling between the two systems. For example, the tightness or looseness of the inter-working between the first and second networks may refer to whether the delay (or latency) between the two networks can be controlled to within some predefined bound. If the bound on the delay is relatively large, the coupling is loose; if the bound on the delay is relatively small, the coupling is tight. Accordingly, tightly coupled systems may provide a “delay-guaranteed” handover and loosely coupled systems may provide a “best-effort” handover.
The tightness or looseness of coupled systems may also refer to the ability of one or both of the systems to direct data from a central point to an end point within the networks. For example, tightly coupled systems may include a routing path that includes a router known to, and controllable by, both systems. In contrast, loosely coupled systems may include a routing path that includes one or more routers that one of the systems has no knowledge of, or control over. The tightness or looseness of coupled systems may also refer to the ability of the systems to perform authentication from the second network back to the first network. For example, authentication of messages in tightly coupled systems may be managed directly because the second network is an extension of the first network, whereas authentication of messages in loosely coupled systems may not be managed directly because the second network is not an extension of the first network. Furthermore, mobility within tightly coupled systems may be managed by a foreign agent that is owned by the operator, whereas mobility within loosely coupled systems may be managed by a foreign agent that is not owned by the operator
The likelihood that a call will be interrupted or dropped during a handover depends, at least in part, on the relative tightness or looseness of the connection between the systems participating in the handover. For example, if the first system is tightly coupled to a second system, the delay and/or latency of messages is relatively small and the likelihood that the call will be interrupted or dropped during the handover is reduced. In contrast, the delay and/or latency of messages may be relatively large and the likelihood that the call will be interrupted or dropped during the handover may be increased if the first system is loosely coupled to the second system. However, tightly coupled systems typically have more complex designs and implementations than loosely coupled systems. Tightly coupled systems may also deviate from conventional standards for telecommunication systems.
The present invention is directed to addressing the effects of one or more of the problems set forth above.